This invention relates to thermoplastic branched polycarbonates of high molecular weight possessing flame-resistant properties.
Polycarbonates are known in the art for their excellent physical and mechanical properties such as their high impact strength and considerable dimensional and thermal stability.
Because of the increasing requirement for materials which for safety reasons possess not only excellent mechanical properties but also flame-resistant properties, various methods have been devised in the art for making polycarbonates self-extinguishing. One of the most commonly used methods is based on introducing halogens, mainly bromine and chlorine, into the polycarbonate. The halogens can be introduced into the polymer in the form of additives by using generally polyhalogenated organic substances such as described for example in U.S. Pat. No. 3,357,942, if desired together with other additives with synergic action, such as antimony oxide (J. T. Howarth et al., Plastic World, p. 64-74, March 1973).
It is also known to chemically bond the halogens to the polymer chain by using bifunctional phenols such as tetrabromobisphenol A and tetrachlorobisphenol A as co-monomers in the preparation of the polycarbonate (U.S. Pat. No. 3,334,154).
Halogenated substances of the known art, whether additives or monomers to incorporate in the polymer chain, must however be used in rather large quantities to give the polycarbonate the required self-extinguishing properties.
Although the presence of large halogen quantities on the one hand makes the polymer able to resist the flame, on the other hand it leads to degradation of the polycarbonate during its working, to thus cause deterioration in the inherent physical and mechanical properties of the non-halogenated polycarbonate.
Moreover, the high temperatures necessary for working the polycarbonate can cause degradation of the halogenated compounds, with release of hydrohalogen acids and consequent damage to the machines by corrosion.
Thus, the technical problem still unsolved is to produce polycarbonates possessing flame-resistant properties but which preserve all their inherent chemical, physical and mechanical properties intact.